The present invention relates generally to a thermal ink jet print head and, more particularly, to a thermal ink jet print head fabricated using a less modified standard bipolar processing technique, integrated all device elements in a single silicon substrate, and having a high heat transfer efficiency.
Thermal ink jet print heads available on the market are commonly manufactured using at least two steps: fabricating a nozzle plate and a heating resistor plate separately and then bonding them together. The whole process involves several processing technologies, including semiconductor processing, glass processing, plastic processing and metal plating.
A typical example is a thermal ink jet print head having a nozzle array partially made of nickel. The head is fabricated in two steps. The first step is to form a nozzle plate and a heating resistor plate separately. In order to form the nozzle plate, a photoresist pattern with a plurality of parallel stripes is formed on a stainless steel plate by a photolithographic process. Using the patterned stainless steel plate as a substrate a nickel layer is formed thereon by electrical plating. In order to form the heating resistor plate a silicon substrate is thermally oxidized in wet oxygen to grow an oxide layer thereon. A conductive layer is deposited on the surface of the silicon substrate and patterned to form a heating resistor array by a photolithographic process. The second step is to bond the two plates together by anodic bonding. In this way the trenches in the nickel nozzle plate are covered to form channels and nozzles, and the heating resistors are arranged so that each resistor is disposed in a channel.
In another typical example a plastic plate is used for constructing a thermal ink jet print head. The plastic plate is formed by thermal plastic press process so that its one surface has a plurality of parallel trenches respectively connecting to a square throughout hole therein. The plastic plate is then bonded to an oxide coated silicon substrate having a plurality of thin film heating resistors thereon by applying a resin adhesive.
In these two types of thermal ink jet print heads, there are many problems remaining to be considered. One problem is that the fabrication process involves not only semiconductor processing technology, but also other totally different processing technologies, such as plastic processing technology and metal processing technology. Another problem is that the bonding of two or more plates together requires plate to plate alignment with high accuracy and additional processing equipment. Still another problem is that the mold formation process restricts the size of the thermal ink jet print heads. Because of these problems, the production cost is still relatively high and the performance has been improving very slowly.
In recent years many efforts have been made to develop monolithic thermal ink jet print heads. In one way, a thermal ink jet print head is fabricated based on bulk anisotropic etching. Ethylene-diamine-pyrocatechol-water (EDP) is used as an anisotropic etchant for single crystal silicon. The etch rates in EDP for (100) and (110) planes are much higher than that for (111) plane. The etch rate in EDP for thermal oxidized oxides is three to four orders of magnitude lower than that for (100) and (110) planes. So EDP can be used to undercut a network of highly-boron-doped silicon support ribs and form an array of microchannels. The etch windows of the microchannel are then sealed using thermal oxidation and LPCVD dielectrics. Polysilicon heating resistors are integrated on the top of each microchannel so that when activated, the underlying ink is vaporized and a drop of ink is expelled from the microchannel.
There are still several problems with this monolithic thermal ink jet print head. One problem is lower heat transfer efficiency due to the two facts: (1) the heat generated by the resistors must pass through a dielectric film with low thermal conductivity on its way to the ink; (2) a part of the heat can flow to the silicon walls through the silicon ribs with high thermal conductivity. Another problem is the thermal cross talk between channels due to the heating resistors located on a thin dielectric film and with a short spacing from each other. Still another problem is the uneven ceiling surface of the micro-channels resulting from sealing the etch windows using thermal oxidation and LECVD dielectrics which may hinder the movement of the ink vapor bubbles.
The present invention is directed to overcome all above-mentioned problems with the multiple-plate structure and the monolithic structure thermal ink jet print heads.
One purpose of the present invention is to provide a thermal ink jet print head having a heating resistor array with each resistor made in a silicon stripe that is surrounded by a thermal isolating material filled trench so that the resistor has a higher lifetime and no cross talk takes place.
Another purpose of the present invention is to provide a thermal ink jet print head having a microchannel array with each microchannel buried under a silicon stripe that has a heating resistor formed therein so that the head generated by the resistor directly flows into the ink disposed in the microchannel.
Still another purpose of the present invention is to provide a thermal ink jet print head having a heating resistor array formed in a silicon epitaxial layer that is commonly used for a standard bipolar integrated circuit fabrication process.
Still another purpose of the present invention is to provide a thermal ink jet print head having a microchannel array totally formed in a silicon substrate.
Still another purpose of the present invention is to provide a thermal ink jet print head with all the device elements and the driving switch circuit integrated in/on a silicon substrate.
Still another purpose of the present invention is to provide a thermal ink jet print head the microstructure of which is fabricated using a porous silicon micromachinig technology that is compatible with the standard TTL or BICMOS fabrication processes.